We are studying the immunobiology of successful cardiac allografts across the major histocompatibility barrier. Lethally irradiated rats, grafted with incompatible hearts, will accept 75% of the transplants if repopulated with bone barrow and thymus cells on day 2. We are investigating the cell which can transfer tolerance from the original host to 700R irradiated host. Adjunct drug therapies are being studied to determine whether they can further improve our results. Since vascular grafts are difficult in mice, we are attemping to adapt our protocol to skin grafts and free pancreatic islet cell grafts in mice so that we might take advantage of the greater amount of knowledge about mouse immunogenetics.